An apparatus for measuring acoustic emission is activated only at critical periods which are determined by detection of the background noise from a reference level. An acoustic emission transducer continuously transmits a signal to a detection circuit which delivers the top modulation envelope of...http://www.google.ca/patents/US4423634?utm_source=gb-gplus-sharePatent US4423634 - Device for the activation of an apparatus for measuring acoustic emission by detection of background noise 
